(a) Field of the invention
The present invention relates to a method of preparing a vegetable oil-modified phenolic resin useful as a material of paper-phenol laminates and copper-clad laminates (hereinafter, they will be given a general name of "laminate") and to a laminate which is produced by using the vegetable oil-modified phenolic resin and is excellent in low-temperature punching quality.
(b) Description of the Related Art
Paper-phenol laminates have been pierced or shaped by punching using pierce dies or blank dies. In order to minimize the dimensional change of printed wiring boards, it has become a new tendency to carry out the punching at room temperature without heating, and flexibilized phenolic resins are used for the production of laminates to improve the punching quality of the laminates. The flexibilization of phenolic resins has mainly been performed by using tung oil, which comprises mainly of eleostearic acid and is very reactive or phosphoric esters, such as triphenyl phosphate. However, tung oil, which occur in nature, is inseparable from severe fluctuation of price and is often hardly available due to natural disasters, such as abnormal weather, floodwater and earthquake, or artificial factors, so that they are difficult to supply stably. Also, the flexibilization by using phosphoric esters causes deterioration in the solvent resistance and electrical properties of laminates, thereby making it difficult to improve low temperature punching quality by using phosphoric esters alone.
Some examples of other vegetable oils which have been examined as the alternatives to tung oil are linseed oil and soybean oil, which comprise mainly of linoleic acid or linolenic acid. These vegetable oils however are less reactive with phenols. In order to solve this problem, it has been proposed to allow such vegetable oils to react with phenols under severe reaction conditions, for example, in the presence of large amounts of Lewis acids [Japanese Patent Application Kokai Koho (laid-open) No. 55-3903 and Japanese Patent Application Kokai Koho (laid-open) No. 55-108414]. However, the use of large amounts of Lewis acids also causes hydrolysis of the vegetable oils and increases low molecular weight components, thereby causing problems in that, after the production of laminates, the low molecular weight components and unreacted vegetable oils ooze out of the laminates and the large amounts of catalysts deteriorate the electrical properties of the laminates.